1. Field of the Invention
The present invention relates to compact zoom lenses for use in photographic cameras, video cameras, electronic still cameras, etc.
2. Description of Related Art
Recently, as the home video camera has been reduced in size and weight, minimization of the bulk and size of the zoom lens, too, has made a remarkable advance, and great efforts are being devoted, in particular, to greatly reduce the total length of the zoom lens, to further reduce the diameter of the front lens members, and to seek a simpler configuration.
To attain these aims, optical system means has been developed to limit the range or zoom ratio to 2 to 3, and to provide a simple arrangement for the zoom lens of 2-unit or 3-unit configuration.
For example, Japanese Laid-Open Patent Applications No. Sho 55-35323 and No. Sho 56-158316 disclose a 3-unit zoom lens comprising, in order from an object side to an image side, a first lens unit of negative refractive power, a second lens unit of positive refractive power and a third lens unit of positive refractive power, the second lens unit axially moving to vary the focal length, while simultaneously moving the first lens unit to compensate for the shift of the image plane.
Such a type of zoom lens in which the front lens unit is negative in refractive power, or the so-called "negative lead" type, is favorable for widening the field angle at the wide-angle end with relative ease. Therefore, it has found its use in many zoom lenses that cover a field of view of 60.degree. or more.
For example, Japanese Patent Publications No. Sho 59-16248 and No. Hei 6-66008 propose zoom lenses having two lens units of negative and positive refractive powers with the separation therebetween varying to vary the focal length, or the so-called "short" zoom lenses.
Also, Japanese Patent Publication No. Hei 7-52256 proposes a zoom lens comprising, in order from an object side to an image side, a first lens unit of negative refractive power, a second lens unit of positive refractive power and a third lens unit of positive refractive power, with the separation between the second and third lens units increasing as zooming occurs from the wide-angle end to the telephoto end.
Also, U.S. Pat. No. 5,434,710 discloses a zoom lens comprising, in order from an object side, a first lens unit of negative refractive power, a second lens unit of positive refractive power and a third lens unit of positive refractive power, with the separation between the second and third lens units decreasing as zooming occurs from the wide-angle end to the telephoto end.
Further, for the zoom lens of a video camera, the improvement of the compact form must be combined with a requirement of achieving a high resolution over the entire zooming range.
In general, in order to increase the resolution, the aberrations every lens unit produces may be minimized. To this purpose, a simple practice is to increase the number of constituent lenses in each lens unit to decrease the contributions of each lens unit to the aberrations. This method, however, increases the lens-system size, contradicting the goal of minimizing the size of the lens system.
In contrast, one of the prior known methods using an aspheric surface corrects spherical and zonal aberrations while minimizing the total number of lens elements. With the use of an aspheric surface, it becomes possible to expect an ever increasing reduction in the number of lens elements and the production of an aberration-correcting effect that cannot be obtained from the spherical surfaces.
Meanwhile, in order to achieve a lens system of high resolving power, it is important to correct chromatic aberrations as well as spherical and zonal aberrations. The above-described method that relies on the asphericity can, however, hardly correct chromatic aberrations.
Particularly for the 3-unit zoom lenses described above, the selection of the second lens unit as the main variator tends to increase the variation of chromatic aberrations with zooming. For this reason, in the prior art, the second lens unit has been achromatized by using a negative lens made from a material of high dispersion and a positive lens made from a material of low dispersion, each one or more in number.
Because of this arrangement, there are fewer degrees of freedom available to the second lens unit to correct aberrations other than chromatic aberrations. Therefore, the difficulty of correcting off-axial aberrations tends to increase at the wide-angle end of the zooming range.
A zoom lens with few members has previously been proposed in, for example, U.S. Pat. No. 4,999,007. In particular, the first and second embodiments disclosed in U.S. Pat. No. 4,999,007 show practical examples with a zoom ratio of no less than 3. However, the first lens unit is constructed with as small a number of members as 1 or 2. So, the aberrations the first lens unit produces, including chromatic aberrations, are not sufficiently corrected. Another problem arises from the fact that the aspherical first lens in the first embodiment is formed to a shape unfavorable for making it by molding. Concretely speaking, the axial and edge thicknesses differ so greatly from each other that the lens, although easy to make, is very hard to handle in detaching it from the mold. Also, in the second embodiment, although the drawbacks described above are slight, the field angle is narrow, as the wide-angle end is not designed to extend toward short enough focal lengths.
In Japanese Laid-Open Patent Application No. Hei 1-191820, too, a zoom lens with few members is proposed, in which embodiments of a zoom ratio of about 3 are disclosed. However, the first lens unit is constructed with one or two members and is not sufficiently corrected for aberrations, including chromatic ones. In addition, the zoom lens is designed not to make the maximum field angle wide enough.
Meanwhile, in Japanese Laid-Open Patent Application No. Hei 6-11650, a negative lens unit at the frontmost position is followed, on the image side thereof, by a positive lens unit. In such a zoom lens, the first lens unit is constructed with either positive and negative lenses, totaling two lenses, or negative and positive lenses, totaling two lenses, or a negative meniscus lens, a bi-concave lens and a positive meniscus lens, totaling three lenses, or a negative meniscus lens, a bi-convex lens and a bi-concave lens, totaling three lenses. The aberrations the first lens unit produces are not sufficiently corrected.
Japanese Laid-Open Patent Application No. Hei 3-240011 discloses a 3-unit zoom lens of a negative-positive-positive refractive power arrangement in this order from the object side to the image side. The first lens unit is constructed with a negative meniscus lens, a bi-concave lens and a positive meniscus lens, totaling three lenses. So, the aberrations the first lens unit produces are not sufficiently corrected.
Another Japanese Laid-Open Patent Application No. Hei 6-94996, too, discloses a 3-unit zoom lens of a negative-positive-positive refractive power arrangement in this order from the object side to the image side. The first and second lens units are both constructed in such a way as not to use any aspheric surfaces. So, the first lens unit produces distortion and, during zooming to the wide-angle region, lateral aberration. The second lens unit produces spherical aberration and astigmatism. These aberrations are not sufficiently corrected.
Yet another Japanese Laid-Open Patent Application No. Hei 8-152558 discloses a zoom lens including negative and positive lens units in this order from the object side to the image side. The second lens unit is constructed in such a way as not to use any aspheric surfaces. So the spherical aberration and astigmatism the second lens unit produces are not sufficiently corrected.
As described before, the negative lead type of zoom lens is favorable for reducing the size of the lens system and widening the field angle. To simultaneously fulfill the requirements of widening the field angle while still assuring improvements of the compact form of the entire lens system, and of obtaining a good image quality over the entire area of the image frame, however, there is a need to appropriately determine the refractive power arrangement of all the lens units and the forms of the constituent lenses. Otherwise, the range of variation of aberrations with zooming would increase greatly, and it would become difficult to obtain images of good quality over the entire area of the image frame.
Meanwhile, as the video camera, digital still camera or like photographing apparatus using a solid-state image pickup element gets ever higher capabilities, there is a growing demand that the optical system to be used is a zoom lens of large aperture ratio with the inclusion of wide field angles. In a camera of this kind, optical elements other than the lens, such as a low-pass filter and color correction filter, are installed in the space between the last lens member and the image pickup element. For this reason, the optical system to be used is required to have a relatively long back focal distance. Further, in the case of the color camera using a set of image pickup elements for color images, color shading must be avoided. For this purpose, the optical system to be used is desired to have its image side exhibit good telocentric characteristics.
A zoom lens configuration in which two lens units, namely a first lens unit of negative refractive power and a second lens unit of positive refractive power, move in differential relation, or the so-called "short zoom" type, has found its use in many wide-angle 2-unit zoom lenses. In these optical systems of the short zoom type, the positive second lens unit is moved to vary the focal length and the negative first lens unit is moved to compensate for the image shift with the variation of the focal length.
These two lens units provide an arrangement that assures a zooming range of about 2. To increase the zoom ratio to more than 2 and still make up the lens totality in a compact form, Japanese Patent Publications No. Hei 7-3507 and No. Hei 6-40170, for example, have proposed that a third lens unit of negative or positive refractive power is added to the 2-unit zoom lens at the image side and used for correcting all the aberrations resulting from the increase of the zoom ratio, that is, the so-called 3-unit zoom lenses.
However, since these 3-unit zoom lenses are designed to be used mainly for 35 mm film photography, it is hard to say that the back focal distance matches the required one for an optical system using a solid-state image pickup element and coincides with good telecentric characteristics as to suffice for that optical system.
3-unit zoom lenses which exhibit the back-focal-distance and the telecentric characteristics and are widened in the field angle are proposed in, for example, Japanese Laid-Open Patent Applications No. Sho 63-135913 and No. Hei 7-261083. Also, in Japanese Laid-Open Patent Application No. Hei 3-288113, there is disclosed an optical system as derived from such 3-unit zoom lenses by making the first lens unit of negative refractive power remain stationary. The second and third lens units of positive refractive power move to effect zooming. In these conventional examples, however, each of the lens units has a relatively large number of constituent lenses, thereby undesirably elongating the total length of the entire lens system and increasing the production cost.
The above Japanese Laid-Open Patent Application No. Hei 7-261083 shows an example of a first lens unit of negative refractive power having a convex lens (positive lens) at the frontmost position. Particularly when widening the field angle, it is, therefore, unavoidable to suffer the drawback of increasing the diameter of the complete lens. Further, in this example, the first lens unit of negative refractive power is made movable for focusing. To control the focusing movement together with the zooming movement, there is another drawback that the operating mechanism becomes complicated in structure.
Also, in the above U.S. Pat. No. 4,999,007, there is disclosed a 3-unit zoom lens with the first and second lens units each in the form of a single lens. However, the total length for the wide-angle end of the entire lens system is relatively long. Moreover, because the first lens unit is spaced from a stop by a very long distance in the wide-angle end, the off-axial rays are incident on the first lens unit at a large height. As constituent lenses of the first lens unit increase in diameter to admit those rays, there is a drawback that the whole lens system becomes bulky.
Further, in a case where the field angle for the wide-angle end is widened, a peculiar problem arises in that distortion is under-corrected. To allow employment of an image pickup element of high pixel density whose sensitivity is relatively low, the relative aperture must be much increased.